Distortion

The DA* 55mm f/1.4 SDM produces only a very minor degree of barrel distortion (0.6%) which is usually nothing to worry about in field conditions.

The chart above has a real-world size of about 120x80cm.

Vignetting

The amount of vignetting is very well controlled. At f/1.4 it is naturally most pronounced at ~0.75EV - this may be visible in very critical situations but it is no worse than comparable lenses here. It is also an indication that the lens covers more than just the APS-C image circle. From f/2 onwards the vignetting is no longer field-relevant.

MTF (resolution)

The DA* 55mm f/1.4 SDM shows the rather typical resolution curve of a standard lens. It is fairly soft (inc. contrast) at f/1.4 and f/2 although the center quality is quite fine here already. There's a substantial boost in quality at f/2.8 where the center reaches an excellent level and the borders and corners deliver very good quality. The peak resolution is reached between f/4-5.6 where it is basically excellent across the board. At f/8 diffraction effects start to lower the resolution potential again (as usual).

The lens showed no significant amount of field curvature.

Below is a simplified summary of the formal findings. The chart shows line widths
per picture height (LW/PH) which can be taken as a measure for sharpness.
If you want to know more about the MTF50 figures you may check out the corresponding
Imatest Explanations

Chromatic Aberrations (CAs)

Lateral chromatic aberrations (color shadows at harsh contrast transitions) are very well controlled at large aperture settings. However, the problem increases the further you stop down although it does never reach really critical levels with a measured maximum of about 1px at the image borders at f/8.

(Note: CAs can be reduced via imaging tools)

Bokeh

The bokeh (the quality of the out-of-focus blur) is a primary aspect for a standard lens and the DA* 55mm f/1.4 SDM does not disappoint us here. There's a bit of an outlining effect in out-of-focus highlights at f/1.4 but highlight discs are pretty perfect at f/2 and f/2.8. The blur quality is also very high although it's a bit nervous in the immediate focus transition zone. All-in-all quite impressive except for the bokeh fringing - see the next chapter.

Bokeh Fringing / Longitudinal Chromatic Aberrations (LoCA)

Bokeh fringing is a common problem present in most large aperture lenses and the Pentax lens is no exception to the rule here. If you look at the provided sample crops below you should be able to spot a purple halo in front of the focus zone and a green one beyond. This is clearly visible at f/1.4 and f/2. At f/2.8 the problem starts to fade and it's more or less resolved at f/4.

Move the mouse cursor over the f-stop marks below to observe the respective LoCAs